package com.itheima.fanxing5;

import java.util.ArrayList;

public class Demo02 {
    public static void main(String[] args) {
        ArrayList<Object> objects = new ArrayList<>();
        ArrayList<Integer> integers = new ArrayList<>();
        ArrayList<Number> numbers = new ArrayList<>();
        ArrayList<String> strings = new ArrayList<>();
        sing1(objects);
        sing1(integers);
        sing1(numbers);
        sing1(strings);
        //封上限
       // sing2(objects);
        sing2(integers);
        sing2(numbers);
        //sing2(strings);


        //封下限
        sing3(objects);
        //sing3(integers);
        sing3(numbers);
        //sing3(strings);
    }


    public static void  sing1(ArrayList<?> list){}
    //封上限
    public static void  sing2(ArrayList<? extends Number> list){}
    //封下限
    public static void  sing3(ArrayList<? super Number> list){}
}
